He would have to be impeached, but with a Dim Congress and Senate, who knows. However, all of the laws he would signed could be disputed in the courts, the military would not have to heed his command, and foreign nations would not know whether his signature on any agreement carried any value what so ever.
The consequences would be grim. (I think this is the reason why the courts shy away. They really do not want to touch this issue.)
Well if his signature is invalid on anything then he’s just a figurehead and OBiden would be running the country ....